Enabling and disabling TCP push preserve

Use the tcp-push-preserve command to enable or disable TCP push preserve in the switch.

Syntax:


no tcp-push-preserve


show tcp-push-preserve

TCP push preserve is disabled on the switch (see the important note below). Use the tcp—push—preserve command to enable it. To disable this feature, use the no form of the command.

This feature is applicable to all slots in a switch or all switches in a stack. Per slot configuration is not supported.
When enabled, the tcp-push-preserve command results in TCP packets with the push bit set are being given priority by buffering them on the ingress packet queue of the receiving port if the output buffer of the packets destination port is full. That way the packets remain in the input queue when the egress queue is congested. In some situations, a large amount of TCP traffic with the push bit set could overwhelm the input queue, causing packet drops and possible network outage.
When disabled (the default), TCP packets flow with the push bit set and are dropped normally when the egress queue is congested.
The show tcp-push-preserve command displays whether the tcp-push-preserve feature is enabled or disabled in the switch.

IMPORTANT:

Earlier versions of switch software had tcp-push-preserve enabled by default. When upgrading to the 16.03 release, the default for tcp-push-preserve will be changed from enabled to disabled. This change was made due to performance considerations in modern networks. A message is logged indicating that the tcp-push-preserve feature was disabled and the default configuration changed. This change was also made to 16.01, and 16.02 releases. In 15.17 and earlier releases, the TCP push preserve feature remains enabled by default.

Changing the TCP push preserve feature to be disabled by default affects how tcp-push-preserve behaves in the switch configuration. You need to be aware of the impact this difference has on activities related to switch configuration. See the sections below for details on the implications of upgrading from, downgrading to, or using a configuration from a 15.17 system — on a switch running a later release where tcp-push-preserve is disabled by default.

Impact of the TCP push preserve feature being disabled by default instead of enabled

The default configuration (tcp-push-preserve disabled) will not show the no tcp-push-preserve line in the show running-configuration output.

Switch# show running-configuration

Running configuration:

; J9575A Configuration Editor; Created on release #KA.16.03.0000x
; Ver #0e:5f.f8.92.34.5f.3c.6b.fb.ff.fd.ff.ff.3f.ef:2e
hostname "HP-8206zl"
module 1 type j9548a
module 6 type j9534a
snmp-server community "public" unrestricted
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit

When the TCP push preserve feature is enabled you will see the tcp-push-preserve line in the show running-configuration output

Switch# show running-configuration

Running configuration:

; J9575A Configuration Editor; Created on release #KA.16.03.0000x
; Ver #0e:5f.f8.92.34.5f.3c.6b.fb.ff.fd.ff.ff.3f.ef:2e
hostname "HP-8206zl"
module 1 type j9548a
module 6 type j9534a
snmp-server community "public" unrestricted
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
tcp-push-preserve

What happens when upgrading to 16.01, 16.02, 16.03 or later versions from 15.17 with the default configuration for the TCP push preserve feature.

The default behavior in 15.17 for TCP push preserve is enabled, and the tcp-push-preserve line is not present in the show running-configuration output. When the switch is upgraded to 16.01, 16.02, 16.03 or later software, the default behavior of this feature is changed to disabled, and the tcp-push-preserve line will not appear in the show running-configuration output. A message will be logged to inform you that the default behavior is changed.

show running-configuration output with 15.17

Switch(config)# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.15.17.0011A
; Ver #0e:01.b3.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:cb
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit

show running-configuration output example from 16.03 after upgrade

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.16.03.0000x
; Ver #0e:3f.bb.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:fd
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
activate software-update disable

Message logged when a switch is upgraded from 15.17 to 16.01, 16.02, 16.03 or later versions

I 10/07/16 14:00:33 05560 system: The tcp-push-preserve feature was disabled. This is a change to default configuration.

What happens when upgrading to 16.01, 16.02, 16.03 or later versions from 15.17 after disabling TCP push preserve feature.

When TCP push preserve feature is disabled in 15.17, the no tcp-push-preserve line appears in the show running-configurationoutput. When the switch is upgraded to 16.01, 16.02, 16.03 or later versions, the default behavior of this feature is changed to disabled and the no tcp-push-preserve line will not appear in the show running-configuration output. A message will be logged to inform you that the default behavior is changed.

show running-configuration output of 15.17 after disabling TCP push preserve feature

Switch(config)# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.15.17.0011A
; Ver #0e:01.b3.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:cb
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
no tcp-push-preserve

show running-configuration output with 16.03 after upgrade

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.16.03.0000x
; Ver #0e:3f.bb.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:fd
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
activate software-update disable

What happens when downloading a configuration file created by 15.17 with the default TCP push preserve configuration to a switch running 16.01, 16.02, 16.03 or later versions.

The default behavior in 15.17 for TCP push preserve is enabled and the tcp-push-preserve line is not present in the show running-configuration output. When the switch is upgraded to 16.01, 16.02, 16.03 or later, the default behavior of this feature is changed to disabled and the no tcp-push-preserve line will not appear in the show running-configuration output. Hence the downloaded configuration file will be discarded as an identical file if all other configurations present in it are matching with what is present in the startup configuration of 16.01, 16.02 or 16.03 image.

After trying to download the configuration
```
Switch# copy tftp startup-config 10.128.0.122 ip_route
Device may be rebooted, do you want to continue [y/n]?  y
TFTP download in progress.
000M Downloaded file identical to current, no update performed.
Switch#
```

What happens when downgrading to 15.17 from 16.01, 16.02, 16.03 or later versions with default configuration for TCP push preserve feature.

The default behavior in 16.01, 16.02, 16.03 or later versions for TCP push preserve is disabled and the no tcp-push-preserve line is not present in the show running-configuration output. When the switch is downgraded to 15.17, the default behavior of this feature is changed to enabled and the no tcp-push-preserve will not appear in the show running-configuration output.

show running-configuration output with 16.03 image before downgrade:

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.16.03.0000x
; Ver #0e:3f.bb.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:fd
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit

show running-configuration output of 16.03 after downgrade:

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.15.17.0011A
; Ver #0e:01.b3.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:cb
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
device-profile name "default-aos-profile"
   exit
device-profile name "default-scs-profile"
   exit

What happens when downgrading to 15.17 from 16.01, 16.02, 16.03 or later versions after enabling TCP push preserve feature.

When the TCP push preserve feature is enabled in 16.01, 16.02, 16.03 or later versions, the tcp-push-preserve line appears in the show running-configuration output. When the switch is downgraded to 15.17, the default behavior of this feature is changed to enabled and the tcp-push-preserve line will not appear in the show running-configuration output.

show running-configuration output with 16.03 image before downgrade:

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.16.03.0000x
; Ver #0e:3f.bb.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:fd
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
tcp-push-preserve

show running-configuration output of 15.17 after downgrade:

Switch# show run

Running configuration:

; J9850A Configuration Editor; Created on release #KB.15.17.0011A
; Ver #0e:01.b3.ff.7c.5f.fc.7b.ff.ff.fc.ff.ff.3f.ef:cb
hostname "HP-Switch-5406Rzl2"
module A type j9536a
module F type j9534a
snmp-server community "public" unrestricted
oobm
   ip address dhcp-bootp
   exit
vlan 1
   name "DEFAULT_VLAN"
   untagged A1-A22,F1-F24
   ip address dhcp-bootp
   exit
device-profile name "default-aos-profile"
   exit
device-profile name "default-scs-profile"
   exit